There is nothing dishonourable in choosing between a bad outcome and a worse one. The risk remains that by intervening we will both widen and intensify the conflict.

Syrian army soldiers are seen deployed in the Jobar neighbourhood of Damascus on August 24, 2013. Photograph: Getty Images.

British military adventurism in the Middle East is invariably disastrous. Yet, following the recall of parliament on 29 August, here we are again, on the verge of another western military intervention in the Levant, a region with a long history of religious and sectarian conflict.

The alleged use of chemical weapons against its own people by the murderous Bashar al-Assad regime was indeed a crime against humanity. No one who has seen photographs, or YouTube footage, of the suffering caused by the attacks – the hollow-eyed children gasping for breath, the terrified screams of mothers, the panic, the mayhem – can be in any doubt that something evil happened in the Damascus suburb of Ghouta on 21 August. Assad has everything to lose and the rebels fighting him have nothing to lose: the combination is deadly, the suffering perpetual.

Intervention has mainly been justified as a necessary response to the Ghouta atrocity. The refrain from the American and British governments is that we cannot allow states to use chemical weapons with impunity. But, even if we assume the regime was responsible for the attack, there is no reason to believe that missile strikes would dissuade Assad from further massacres. Should his chemical weapons capacity be disabled, the Syrian despot will merely resort to other means by which to murder his opponents.

For this reason, as with Libya, regime change may become the de facto objective of the coalition against Assad. It is far from clear that the consequences would be benign, however. The rebels – a mixed group of genuine democrats, deserters from the Syrian army, Sunni insurgents and al-Qaeda-affiliated Islamist fanatics such as Jabhat al-Nusra – are not a unified and coherent force. They are factionalised, united only in their hatred of Assad and his fellow Alawites, who are a heterodox Shia sect. If the regime is toppled, secular liberal democrats will not replace it – as the west once hoped. The Syrian civil war is both a local and an intra-Islamic conflict, and its aftershocks will reverberate long into the future.

The Israelis and Turks look on anxiously, wondering if and when they will be drawn directly into the conflict. The Kurds are newly emboldened that before too long they will have their own nation state, carved out of parts of Turkey, Iraq and Syria. What we are witnessing is the unravelling of the 1916 Sykes-Picot Agreement and, surely also before too long, the inevitable break-up of the artificial nation states created by Britain and France: Iraq, Syria, Lebanon and Jordan.

Barack Obama’s non-interventionist policy of drawing up a “red line” on chemical weapons, beyond which Assad was forbidden to go, while hoping for the best, has failed. The president is no dove: as our correspondent John Bew – author of a book on Lord Castlereagh and a war studies and foreign policy expert – writes on page 20, he is a realist. But he wanted to avoid being sucked into the Syrian civil war at the very moment US troops are preparing to leave Afghanistan.

Now, after equivocating for so long, President Obama is acting in haste, encouraged by the fading post-colonial powers Britain and France. Instead of preparing for military strikes against Syrian “strategic targets”, the US, the UK and France should do anything possible to keep diplomatic lines open to the Russians. Unilateral military action will close off that avenue. Russia, given its problems with Muslim minorities in the Caucasus, is desperate not to allow Syria to fall into the hands of Islamists. It therefore has every reason to support a peace conference and to force Assad to attend. Assad has rejected this option before but may yet back down under Iranian and Russian pressure. It is clear that he is not going to achieve a quick victory and that the civil war has, in effect, reached a stalemate. At best for him, Syria will be partitioned but he will retain power, however diminished.

There is nothing dishonourable in choosing between a bad outcome and a worse one. The risk remains that by intervening we will both widen and intensify the conflict, and increase rather than reduce the threat to civilian life. Syria and its Lebanese proxies, Hezbollah, might well seek to retaliate with strikes against Israel and Turkey as well as on western forces, creating conditions for the regional conflagration that policymakers have long regarded as the nightmare scenario.

The moral case for action against Assad is indisputable. But it is not enough for leaders to consider what is ethical; they must also consider what is prudent and wise. As the baleful consequences of the interventions in Afghanistan, Iraq and Libya have demonstrated, too often high-minded liberals who go to war intent on preserving civilian life achieve the reverse.

Peace, as such, in Syria is unrealistic but a ceasefire, with all sides keeping what they hold, may be just about achievable. On the principle that war is justified only when all other options are exhausted, this should at least be tried.